The present invention relates to injection molding machines. More particularly, the present invention relates to the common control of multiple shooting pots in a injection molding machine.
Employing control units, such as shooting pots, to introduce thermoplastic resins or other materials into a mold cavity in an injection molding machine is well known. Generally, a primary resin source feeds the material to a shooting pot reservoir which is, in turn, operated to feed a measured, or metered, quantity of the material into the mold cavity. U.S. Pat. No. 3,516,123, entitled xe2x80x9cInjection Molding Machinexe2x80x9d, to Lang; and U.S. Pat. No. 3,231,656, entitled xe2x80x9cApparatus and Method of Plastic Moldingxe2x80x9d, to Ninneman both disclose the use of shooting pots to provide accurately metered shots of resin to a mold cavity. Metering permits an accurate amount of material to be injected into a mold to ensure that a properly formed part is created and to prevent waste of material in the form of xe2x80x9cflashxe2x80x9d, etc. due to overfilled molds. Metering is generally achieved by controlling the distance by which an injection plunger in the shooting pot is retracted and advanced for each shot.
Other metering techniques are also well known. For example, U.S. Pat. No. 4,966,545, entitled xe2x80x9cStaged Shooting Pot for Injection Molding, to Brown, shows how a single shooting pot can be operated to cause two sequential metered injections of the same resin into the same mold cavity. U.S. Pat. No. 4,460,324, to Van Appledorn, entitled xe2x80x9cShot Cylinder Controller for Die Casting Machines and the Likexe2x80x9d, shows how the injection speed of the piston of shooting pot can be controlled, thereby controlling the rate of injection of the resin into the mold cavity.
It is also well known to supply thermoplastic material to a multicavity, mold through a hot runner system. The hot runner system can include a plurality of shooting pots, with at least one shooting pot associated with each mold cavity.
Hot runners systems can also be used for multimaterial injection, or coinjection, molding. Typically, two or more resins are injected, either simultaneously or sequentially, into each mold cavity to produce multi-layered molded structures. For example, a common application for multimaterial molding is the production of food quality containers from recycled plastic. Government standards require that any surfaces which contact the food be made of new, virgin, plastic. To take advantage of lower cost recycled plastics, manufacturers use coinjection techniques to encapsulate recycled material in a sheath of new plastic. U.S. Pat. No. 5,098,274 to Krishnakumar, entitled xe2x80x9cApparatus for Injection Molding of Multilayer Preformsxe2x80x9d, and U.S. Pat. No. 4,717,324 to Schad, entitled xe2x80x9cCoinjection of Hollow Articles and Preformsxe2x80x9d both disclose injection molding machines for multimaterial applications.
Generally, individual control of the shooting pot strokes is provided in these prior art injection molding machines. Separate hydraulic actuation cylinders for each shooting pot injection plunger are mounted inside the machine""s stationary platen. These hydraulic cylinders must be individually set for stroke to control the individual metering of the resins into the mold cavities. The setting of the cylinders can be a hazardous operation, which is performed manually and requires personnel to reach into the machine amongst the heated injection nozzles, close to hot surfaces and heated injection materials. Furthermore, the molding process has to be interrupted for this adjustment, which can cause significant loss of production time, especially in larger machines having up to ninety six injection plungers.
U.S. Pat. No. 4,632,653 to Plocher, entitled xe2x80x9cPress with a Plurality of Injection Plungersxe2x80x9d describes a common actuator for the injection plungers in a transfer molding machine. The injection plungers are actuated by a hydraulic drive acting on a single crosspiece. However, the shooting pot actuator disclosed in Plocher has several limitations and disadvantages which make it inapplicable to metered injection molding machines. Firstly, the shooting pots in a compression molding machine do not provide metered shots. Instead, each shooting pot is filled with an approximate amount of resin and the injection pistons are actuated by the crosspiece to compress the resin into the mold cavity. Plocher discloses pressure compensating pistons and overflow channels to relieve the mold cavities in the case of overfilling, which results in non-uniform product and flashing. Also, there is no mechanism provided for adjusting the stroke of the injection pistons since precise control of the amount of resin injected into the mold is not critical in such a transfer molding process. Second, the crosspiece actuator in Plocher is located within the mold, which increases the cost of designing and manufacturing the mold. Also, such a design is impractical in machines with high clamp forces as the volume occupied by the crosspiece reduces the strength of the mold component in which it is located, thus increasing the likelihood of deformation of mold components when clamped. Further, the mold must be completely disassembled to obtain access for maintenance, adjustment, or replacement.
It is an object of the present invention to provide a novel shooting pot actuator for a multicavity injection molding machine which obviates or mitigates at least one of the disadvantages of the prior art.
In a first embodiment of the present invention, there is provided an injection molding machine comprising:
a clamp unit for clamping a mold having at least two shooting pots each having an injection plunger, said clamp unit including a stationary platen and a movable platen disposed on opposing sides of said mold;
an injection unit to provide to said shooting pots material to be injected;
a shooting pot actuator, exterior to said clamping unit and extending through one of said platens; and
a drive means operable to move said actuator between a first position and a second position, wherein in said first position said injection plungers limit the volume of material which each said shooting pot can receive from said injection unit, and wherein said material is expressed from said shooting pots as said actuator is moved to said second position.
In a further aspect of the present invention there is provided a multimaterial injection molding machine comprising:
a mold having at least two mold cavities, each of said at least two mold cavity having at least a first and a second shooting pot communicating therewith, said first and second shooting pots having respective first and second injection plungers;
a clamp unit including a stationary platen and a movable platen disposed on opposing sides of said mold;
an injection unit to provide to said shooting pots material to be injected;
a shooting pot actuator, exterior to said clamping unit and extending through one of said platens; said actuator having a first group of pushers for abutting said first injection plungers, and a second group of pushers for abutting said second injection plungers; and
a drive means operable to move said first and second groups of pushers between a first position and a second position, wherein in said first position said injection plungers limit the volume of material which each said shooting pot can receive from said injection unit, and wherein said material is expressed from said shooting pots as said actuator is moved to said second position.
In another aspect of the present invention, there is provided a shooting pot actuation assembly for an injection molding machine having a clamp unit for clamping a mold having at least two shooting pots each having an injection plunger, said clamp unit including a stationary platen and a movable platen disposed on opposing sides of said mold, and an injection unit to provide to said shooting pots material to be injected, comprising:
a frame securable to an exterior of one of said platens and having a portion spaced from said platen;
a shooting pot actuator, supported for linear movement within said frame, for extension through said platen to abut said injection plungers; and
a drive mounted on said portion, said drive means being operable to move said actuator between a first position and a second position, wherein said first position determines the volume of material which each said shooting pot can receive from said injection unit, and wherein said volume is expressed from said shooting pots as said actuator is moved to said second position.
In a further embodiment of the present invention, there is provided a shooting pot actuator for a multimaterial injection molding machine having a clamp unit including a stationary platen and a movable platen disposed on opposing sides of a mold having at least two mold cavities and at least first and second shooting pots for each said mold cavity, said shooting pots having corresponding first and second injection plungers, and an injection unit to provide to said shooting pots material to be injected, comprising:
at least two first pushers, each said first pusher operable to abut a respective first injection plunger; and
at least two second pushers through which said first pushers extend, each said second pusher operable to abut a respective second injection plunger;
said first and second pushers operable to move independently between a first and second position wherein said first position determines the volume of material which each respective shooting pot can receive from said injection unit, and wherein said volume is expressed from said shooting pots as said pushers are moved to said second position.